The present invention relates to a driving device using piezoelectric elements, for example., and particularly relates to a driving device suitable for driving an XY stage, photographic lenses of a camera, projection lenses of an overhead projector, lenses of binoculars, and the like.
Conventionally, e.g., in Japanese Patent Laid-Open Publication No. 2000-350482, a piezoelectric actuator 1 as shown in FIG. 7 has been disclosed. In the piezoelectric actuator 1, a bridge circuit is composed of a piezoelectric element 2 and four FETs (field effect transistors) 4, 6, 8, and 10 connected in series, and signals are inputted from a control circuit 12 into bases of the FETs 4, 6, 8, and 10. A power supply 14 is connected between the FETs 4 and 6, and a ground is established between the FETs 8 and 10.
Among the four FETs 4, 6, 8, and 10, the FETs 4 and 6 are p-channel FETs, which are driven into cutoff state when signals inputted from the control circuit 12 into the bases are at high level and are driven into conducting state when the signals are at low level. By contrast, the FETs 8 and 10 among the four FETs 4, 6, 8, and 10 are n-channel FETs, which are driven into conducting state when the signals inputted from the control circuit 12 into the bases are at high level and are driven into cutoff state when the signals are at low level.
FIG. 8 is a timing chart representing an operation sequence of the piezoelectric actuator 1 and shows gate voltage in the FETs 4, 6, 8, and 10 and drive voltage that is applied to the piezoelectric element 2. In a term 1 in FIG. 8, the p-channel FET 6 is in cutoff state with a high signal of H(V) inputted into the gate, the n-channel FET 10 is in conducting state with a high signal of H(V) inputted into the gate, the p-channel FET 4 is in conducting state with a low signal of L(V) inputted into the gate, and the n-channel FET 8 is in cutoff state with a low signal of L(V) inputted into the gate. In this situation, drive voltage of E is applied from the power supply 14 through the FETs 4 and 10 in conducting state to the piezoelectric element 2.
In a term 2 in FIG. 8, the p-channel FET 6 is in conducting state with a low signal of L(V) inputted into the gate, the n-channel FET 10 is in cutoff state with a low signal of L(V) inputted into the gate, the p-channel FET 4 is in cutoff state with a high signal of H(V) inputted into the gate, and the n-channel FET 8 is in conducting state with a high signal of H(V) inputted into the gate. In this situation, drive voltage of −E is applied from the power supply 14 through the FETs 6 and 8 in conducting state to the piezoelectric element 2.
By such alternation of the term 1 and the term 2 in FIG. 8, AC voltage having an amplitude of 2E(V) twice that of power-supply voltage of E(V) is applied to the piezoelectric element 2.
FIG. 9 is figure showing principle of operation of the piezoelectric actuator 1. One end of the piezoelectric element 2 is fixed to a supporting member 16. To the other end of the piezoelectric element 2 is fixed a driving shaft (a driving member) 18 shaped like a round bar, for example. On the driving shaft 18 is movably held a movable member 20. The movable member 20 is engaged with the driving shaft 18 with a predetermined frictional force by virtue of a biasing force of an elastic member not shown such as plate spring and coiled spring. On the movable member 20 are mounted lenses or the like that are objects to be driven and that are not shown.
FIG. 10 shows axial displacement of the driving shaft 18 on condition that drive voltage shaped like rectangular wave as shown in FIG. 8 is applied to the piezoelectric element 2. The axial displacement has a sawtoothed shape with gentle slopes in rising parts and with steep slopes in falling parts. Situations A, B, and C in the drawing correspond to situations A, B, and C in FIG. 9, respectively. When the piezoelectric element 2 slowly elongates from the situation A as an initial situation, the driving shaft 18 and the movable member 20 engaged frictionally therewith are displaced together to a situation B at a comparatively slow velocity. When the piezoelectric element 2 contracts subsequently and rapidly, the displacement of the driving shaft 18 reverts at a comparatively high velocity. Therefore, a slip occurs between the movable member 20 and the driving shaft 18 and results in the situation C in which the movable member 20 has reverted only by a small amount. In the situation C, the movable member 20 has been displaced only by a small amount in an extending direction (a direction in which the movable member 20 goes away from the piezoelectric element 2) in comparison with the situation A that is the initial situation. Repetition of such elongation and contraction of the piezoelectric element 2 causes the movable member 20 to move in the extending direction.
Contrarily, the movable member 20 moves in a returning direction (a direction in which the movable member 20 nears the piezoelectric element 2) according to a principle opposite to the above. That is, displacement of the driving shaft 18 that results from repetition of rapid elongation and slow contraction of the piezoelectric element 2 has a sawtoothed shape with steep slopes in rising parts and with gentle slopes in falling parts, which shape is a reversal of that shown in FIG. 10. Thus a slip occurs between the movable member 20 and the driving shaft 18 when the piezoelectric element 2 rapidly elongates, and the movable member 20 is displaced only by a small amount in the returning direction when the piezoelectric element 2 slowly contracts. Repetition of such elongation and the contraction of the piezoelectric element 2 causes the movable member 20 to move in the returning direction.
FIG. 11 shows a frequency transfer characteristic of velocity of the driving shaft 18 with respect to voltage inputted into the piezoelectric element 2. The velocity of the driving shaft 18 has the characteristic in which the velocity increases in direct proportion to frequency of the voltage inputted into the piezoelectric element 2 on condition that the frequency is comparatively low, in which a primary resonance frequency f1 and a secondary resonance frequency f2 result in high velocities, and in which the velocity has a tendency to decrease on condition that the frequency is higher than the second resonance frequency. In order to obtain such sawtoothed displacement of the driving shaft 18 as shown in FIG. 10 from the drive voltage that is shaped like rectangular wave as shown in FIG. 8 and that is inputted into the piezoelectric element 2, as described in Japanese Patent Laid-Open Publication No. 2001-211669 in accordance with another patent application of the applicant of the present application, it is desirable to set a frequency f of the drive voltage 0.7 times the primary resonance frequency f1 and to set a duty ratio of the drive voltage at 0.3 for driving the movable member 20 in the extending direction (at 0.7 for driving the movable member 20 in the returning direction).
Though the prior art described above makes it possible to drive the piezoelectric actuator 1 with use of the simple driving circuit, the prior art has a problem of low velocity of the movable member 20. The velocity of the movable member 20 can be increased by adjustment of a relation between phases of primary drive frequency component fd and secondary drive frequency component 2*fd that are contained in the drive voltage, with increase in the frequency f of the drive voltage (in this case, the drive voltage is not shaped like rectangular wave).
With reference to FIG. 11, velocity amplitude can be increased with fd and 2*fd nearing f1 and f2, respectively. Provided that the drive voltage is shaped like sawtooth with fd equal to f1 and with 2*fd extremely near to f2, for example, displacement of the driving shaft 18 is also shaped like sawtooth and the velocity of the driving shaft 18 is made higher than usual, as shown in FIG. 11. Thus the velocity of the movable member 20 can be increased.
In this case, however, the resonance frequencies vary according to initial dispersion of the piezoelectric actuator, environmental temperature and the like, and it is therefore impossible to make fd equal to f1 at all times. Accordingly, it has been difficult to achieve stable increase in the velocity against dispersion resulting from mass production, environmental fluctuation, and the like.